The lateral stress induced upon soil subjected to compressive forces is an important and fundamental aspect of engineering behavior of the soil particularly in relationship to the stress on retaining walls and to the soil-bearing capacity under loads. Being able to monitor the ratio of lateral stress to vertical stress is important when structures are constructed upon the soil. Being able to assess this ratio and to evaluate its effect for a given soil sammple is helpful in determining whether or not the soil can properly withstand applied vertical pressure. The ratio of lateral to vertical stress is known as the Rankine stress ratio and is designated "K".
An example of a prior device for measuring lateral pressure induced on the soil by a vertically applied compressive force is shown in U.S. Pat. No. 4,122,704. This device is limited to performing only one soil test, the standard lateral stress test, which measures continuously increasing lateral pressures on the soil when that soil is subjected to various vertical pressures. The device of U.S. Pat. No. 4,122,704 can not perform other tests such as the constant lateral stress test and the constant lateral strain test. These two additional tests are important to an effective analysis of the stress-strain characteristics of the soil.
The constant lateral stress test involves keeping the lateral pressure constant during exposure of the sample to a number of different vertical pressures. Keeping the horizontal pressure constant permits measurement of the horizontal displacement of the soil sample in response to various vertical pressures. The device of U.S. Pat. No. 4,122,704 does not include means for maintaining a constant horizontal pressure in response to various vertical pressures.
Similarly, the device of U.S. Pat. No. 4,122,704 does not provide means for holding the soil sample against horizontal displacement during exposure to various vertical pressures. This capability is essential to performing the constant strain test where during exposure to various vertical pressures the lateral displacement is held constant and measurements of lateral pressures are taken.
Therefore, a primary objective of the present invention is the provision of an improved device for determining the stress-strain relationships of soil.
A further objective of the present invention is the provision of a device capable of yielding accurate results when performing constant lateral strain tests, constant lateral stress tests, and standard lateral stress tests.
A further objective of the present invention is the provision of an apparatus for testing the stress-strain characteristics of soil which may be used either on undisturbed field samples or soil samples which have been subjected to pre-treatment in order to simulate anticipated environmental conditions.
A further objective of the present invention is the provision of a soil testing device which is portable so that the test may be conducted at a field site on samples obtained therefrom.
A further object of the present invention is the provision of a device which is economical to manufacture, durable in use, and efficient in operation.